steady_clock is a monotonic clock: its value never decreases. This can be compared to std::chrono::system_clock, which can in fact decrease if the user changes the value. There's also std::chrono::high_resolution_clock which may or may not be monotonic (as the name suggests, it is intended to primarily be used as a high resolution clock, i.e. ideally with nanosecond precision).

The start_time is a timepoint, and taking the difference of two timepoints (the two now()s) yields a duration. We then convert the duration into millisecond "ticks", which are obtained using count(). Alternatively, we can convert the duration into microsecond "ticks" (as shown above), or even nanosecond ticks. Finally, we refactor the numerical value from milliseconds to seconds (this step is obviously not needed, but is used because it is many times easier to use SI units).